1. Field of the Invention
The present invention relates to a chemical precipitation method for precipitating anions such as nitrate ion, sulfate ion, chloride ion and phosphate ion contained in underground water, surface water or waste water, as insoluble complex salts by reacting the anions with alumina cement and a calcium compound, and to a continuous process for removing the anions.
2. Description of the Background Art
Generally, a compound containing nitrogen and phosphorous has been identified as a substance to accelerate growth of plants, and thereby to become a main element accelerating eutrophication of rivers, lakes and seas. Therefore, researches into ways of removing nitrate ion and phosphate ion in underground water, surface water or waste water have been performed in earnest since 1960""s. Especially, researches for removing nitrate ion which has been known as causing cyanosis of infants have been highlighted in water-treatment field recently.
In the waste water having high nitrate ion concentration, its concentration generally can be lowered by being diluted with water having lower nitrate ion concentration. However, when the water having lower nitrate ion concentration is difficult to use, a reaction for removing nitrate ion is needed. Examples of techniques used for removing nitrate ion include ion-exchange extraction, biological denitration, chemical reduction and electric dialysis, etc. Among them, the ion-exchange extraction and the biological denitration can be actually used for large-scale water treatment.
The ion-exchange extraction is a physical and chemical treatment process requiring reproduction of resin regularly. Sodium chloride or sodium bicarbonate is used for reproducing used-up resins, and consequently, the waste solution concentrated with high concentration of nitrate ion, sodium chloride or sodium bicarbonate should be re-treated or wasted. However, it is difficult to waste the waste solution excessively due to interests and regulations on the environment. Therefore, it is difficult to apply the above process widely. Another problem is in that it is difficult to remove nitrate salt selectively from the underground water which contains a lot of other anions besides nitrate ion. For example, when nitrate ion, sulfate ion, chloride ion, etc. exist simultaneously, it is difficult to remove nitrate ion only, and therefore, nitrate ion removing efficiency is lowered. Especially, when sulfate ion and nitrate ion are exchanged with the chloride ion existed in the resin, the amount of chloride which may cause corrosion of a pipe is increased in the treated water. Occasionally, the amount of chloride may exceed a criterion of chloride content, 200 mg/liter, in water which can be used as a drinking water. Besides, the amount of sodium chloride which is used for reproducing the ion exchange resin is increased, thereby to cause another environmental problem. That is, although the ion-exchange extraction is the best method in view of cost, due to the difficulties in processing concentrated byproducts, such method can be applied only in a seashore area or in an area having no possibility of eutrophication. Also, since a lot of salts should be added, the operational cost is increased. In addition, high concentration of chloride ion in the treated water may cause corrosion.
The biological denitration has been known as a method producing no byproduct and as an economic method, and therefore, used for purifying waste water or outflow of septic tank. This method uses a principle that denitration bacteria reduces nitrate ion to nitrogen gas when an organic substrate is present. The substrate widely used in this method is methanol which has low price and high efficiency. However, in this method, time required for growing up of the bacteria and temperature of reactor affect greatly on the anion removing efficiency. In addition, it is difficult to control conditions since methanol has to be supplied carefully so as to be used relevantly. And blockage due to the growth of the bacteria may happen regularly. Also, there are many problems to be solved such as post-process, problems caused by carbon source and a necessity of sensitive control.
Besides the above methods, chemical reduction of nitrate ion with ferrous hydroxide has been mainly researched in the U.S.A. Ferrous sulfate has been known as the most economic form among various substances having a reducing ability. 1-5 ppm of copper or silver should be used as a catalyst for reducing nitrate ion with a metallic iron. The largest amount of nitrogen gas is obtained at pH 8.0, and remainder is reduced to ammonia. However, this method is disadvantageous in that complete reduction of nitrate ion to nitrogen gas is difficult to be achieved, and metallic iron has to be used too much.
As a method for removing anions containing nitrate ion more economically by solving the problems of the prior art, the present inventors have suggested a chemical precipitation method using alumina cement and burned lime (Korean Patent No. 242994). According to this method, hydration rate of alumina cement can be lowered by adding a small amount of sulfate salt in a process of complexing the alumina cement and the burned lime with nitrate anion, by which the amount of alumina cement wasted by the hydration reaction is reduced and the amount used for forming complex salt is increased. Consequently, the amounts of the alumina cement and the burned lime used can be reduced. Therefore, this method is more economic method for removing nitrate ion than other methods without using sulfate. However, in spite of the above improvements, it is discovered that additional reduction of the amount of materials used in chemical precipitation method such as the alumina cement and the burned lime is required in order to ensure economic efficiency for a commercial use.
The complexation of a calcium compound such as alumina cement and burned lime or hydrated lime with an anion has been known to be competitive with the hydration of the calcium compound with a water molecule. That is, a position which can be reacted in the structure of the alumina cement can be competitively occupied by an anion or a water molecule to react. If the reaction with water molecules is relatively superior to that with anions, the efficiency of the complexation of the anion is lowered. Therefore, in order to increase the efficiency of the complexation with anions, the activity of the anions should be increased to an appropriate level so that the complexation can ensure relatively superior position.
Therefore, an object of the present invention is to provide a method for removing anions contained in underground water, surface water or waste water through a chemical precipitation method for anions which is able to increase an efficiency of anion complexation and to reduce the amount of materials used greatly.
Another object of the present invention is to provide a continuous process for removing anions using the above chemical precipitation method.
The above and other objects of the present invention can be achieved by providing a chemical precipitation method, in which an electric field is applied to a complexation reactor so as to increase the activities of anion and calcium ion in the complexation reactor to an appropriate level, thereby to improve efficiency of anion complexation as twice as that of the prior art.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.